Seat belt restraint systems have been developed over many years and now include retention devices operable, when actuated in response to a sudden, rapid deceleration of the vehicle, usually a frontal crash, to take up slack in the seat belt and maintain the seat belt tightly engaged with a seated person. Such retention devices have been incorporated into a retractor at one end of the seat belt or into a buckle at the opposite end of the seat belt. However, due to the overall length of a seat belt from the retractor to the buckle, and the existence of slack throughout that length, it takes too much time after triggering of the retention device to take up all of the slack. This holds true even though only a fraction of a second is required to actuate the retention device. When a frontal crash occurs, the seated person is not completely prevented from shifting until all of the slack is eliminated.